PLACENTA IN PERAMELES 131 



which are either primarily primitive (as yet unknown to us) 

 or secondarily simplified (Ungulates, Lemurs, Cetacea, 

 &c.) ' (1909, p. 117). 



With similar material before each of them hardly could two 

 authors come to more different conclusions. Whereas Hill is 

 of the opinion that there is no interfusion of maternal and 

 foetal tissues other than that caused by the intergrowth 

 of allantoic tissue— which is comparatively insignificant — 

 Hubrecht is just as emphatic that an invasion of the maternal 

 syncytium by the foetal chorion does occur and that therefore 

 the type of placentation in Perameles corresponds closely 

 with that of some of the Eutheria, notably the Gar- 

 ni a' o r a . 



Interpretation of Professor Hill's Figures : — 



Without as yet adducing any evidence from my own material, 

 I may be permitted to say that from the evidence of Hill's 

 figures alone there appears to be a considerable defence for 

 Hubrecht "s standpoint in this matter. 



According to Hill's account (1897, 1899) the histology of the 

 placental area is extremely simple. With the exception of the 

 syncytial nuclei, endothelial nuclei, and leucocytes, the only 

 elements of this region are the cells of the foetal chorion, 

 which in no case enter into the constitution of the placental 

 thickening but degenerate and disappear. 



With this interpretation before us it will be possible to 

 proceed to examine his figures, particularly those of Pis. xxix 

 and XXX, figs. 7, 8, and 9 (1897), representing portions of the 

 placental area with the chorionic ectoderm attached. These 

 figures are all representative of Stage C (7 mm.). 



As Hill has shown, there is a marked difference in the condi- 

 tion of the chorionic layer dependent more or less on its distance 

 from the centre of the fixed area. In general terms it may 

 be stated that the farther from the centre of the attached area 

 the less alteration is evident. 



Examining PI. xxix, fig. 7, representing the edges of the 

 placental area of Stage C, it will be seen that the ectoderm is 



